51 Residual Astigmatism with Toric Intraocular Lenses A toric intraocular lens (IOL) during cataract surgery results in near emmetropia in the nearly one third of patients with significant corneal astigmatism.1 Early toric designs proved to be rotationally unstable, rotating more than 10 degrees in 50% of cases and more than 30 degrees in 20% of cases.2 Since, advancements in design have allowed for more rotational stability (Table 51.1).3–6 Damage control, though, is needed in the occasional patient who experiences residual astigmatism and is dissatisfied with the results. Surgeons then need to analyze the residual astigmatism, the underlying cause, and how to move forward with the next steps to get patients back on the road to spectacle independence. This chapter provides pearls to prevent this problem and pearls for damage control when problems occur. Residual astigmatism occurs when an implanted toric IOL is not in the ideal position or does not have the correct power and therefore does not neutralize the corneal astigmatism. There are many questions to consider when determining the cause of residual astigmatism: The vision goals of the patient are very important to understand. Toric IOLs are generally intended for patients with regular astigmatism who desire greater spectacle independence for distance vision, yet patients may have the goal of monovision with near vision in one eye and also benefit from reduced astigmatism. In Ahmed et al’s4 2010 study, 54 of 78 patients (69%) with bilateral toric implants were spectacle independent for distance vision. Toric IOLs available in the United States enable correction of regular anterior corneal astigmatism between 0.75 and 4.1 D after astigmatic changes from the surgical incisions. Although astigmatism greater than 4.1 D can be reduced with a single implanted toric IOL, residual astigmatism should be expected. The patient’s postoperative vision goals may not necessarily include reduction of dependence on glasses. The increase in cost, extra tests, and surgical procedures to obtain spectacle independence may not be worth it for these patients, and glasses may be preferred. Toric IOLs do not eliminate irregular corneal astigmatism. For example, patients with ectatic diseases such as keratoconus, or other nonectatic irregular astigmatism, such as prior refractive surgery, keratoplasty, corneal scars, significant anterior basement membrane dystrophy, or Salzmann’s nodular degeneration, often have poorer results with toric IOL implantation. Although case studies have demonstrated the benefit of toric IOLs in some eyes with keratoconus7,8 and pellucid marginal degeneration,9 it is important to note the likelihood of residual irregular astigmatism or unpredictable results. In ectatic irregular astigmatism cases, it is also important to assess the risk of disease progression. Additionally, because there is now intraocular toricity in addition to the irregular corneal astigmatism, specialty contact lens fitting, if necessary, would be more difficult and likely require a front toric contact lens. Patients with advanced macular degeneration, glaucoma, or amblyopia may not have enough improvement in vision to justify the cost of a toric IOL. Pseudoexfoliation syndrome, phacodonesis, prior vitrectomy, or other causes of lax zonules can be relative contraindications of toric IOL implantation. Abnormal capsular integrity decreases the IOL stability and therefore increases the probability of a misaligned toric IOL. Discontinuation of contact lens use and upright head positioning during testing and marking facilitate making accurate measurements. Using two or more confirmatory keratometry sources such as topography, optical biometry, or manual keratometry is important because there can be variability in measurements (Box 51.1). The topography is also used to assess the regularity of the astigmatism, as keratometry can often miss irregular astigmatism. It is also important to note the opportunity for human error when transcribing measurements and using toric IOL calculators. When using a toric calculator to determine the correct toric IOL, it is important to consider the estimated residual astigmatism given with the calculations. Because this is the amount of astigmatism that can be expected if the IOL is in the optimal position and corneal astigmatism is unchanged after surgery, it should be used as a baseline when predicting the potential amount of residual astigmatism. There are many toric IOL calculators, but the one that we find most useful preoperatively is the Barrett Toric Calculator that is now available on the American Society of Cataract and Refractive Surgery (ASCRS) Web site (http://www.ascrs.org/barrett-toric-calculator). Many methods have been described for determining the axis of alignment. These range from simple marking of the ocular meridians and then aligning a secondary device intraoperatively to intraoperative photographic comparisons, to intraoperatively measuring preinsertion and postinsertion ocular astigmatism10–12 (Figs. 51.1 and 51.2). Fig. 51.1 Intraoperative guidance via the Verion device (Alcon, Fort Worth, TX). Accurate determination of the surgically induced astigmatism can improve the accuracy of toric IOLs.13 The amount of SIA induced depends on the size and location of the incision and the biomechanical properties of the cornea. Warren Hill has developed the Surgically Induced Astigmatism Calculator (http://www.sia-calculator.com/) to help surgeons calculate the SIA. About 9% of eyes have greater than 0.50 D of posterior corneal astigmatism, and if not taken into consideration it can lead to the overcorrection of with-the-rule (vertical steep meridian) anterior corneal astigmatism and undercorrection of against-the-rule (horizontal steep meridian) anterior corneal astigmatism14,15 (Fig. 51.3). Treatment methods of residual astigmatism are as follows (also see the treatment algorithm in Fig. 51.4): If the toric IOL is not aligned with the corneal astigmatism, its ability to correct corneal astigmatism is diminished by about 3.3% for every degree of mismatch between the corneal astigmatism and the IOL toricity. If the IOL is 90 degrees away from the ideal axis, the astigmatism is doubled.16 Table 51.2 shows this loss of function and how the absolute loss is greater in IOLs with higher power (Fig. 51.5). A misaligned toric IOL can leave postimplant patients unsatisfied with their uncorrected distance vision acuity because of residual astigmatism. When this happens, surgeons should consider rotating the toric IOL to its optimal position. In doing so, several questions can help determine the course of action. It is important to check the current axis of the toric IOL. Although the normal angular markings on a slit lamp typically do not offer the precision necessary to determine the exact IOL location, there are a few methods that can be used.
Residual Astigmatism
Successful Implementation of Toric IOLs
Choose Patients Who Desire Spectacle Independence
Choose Patients with Regular Astigmatism
Reasons that Problems Occur
Patients Who Do Not Value Spectacle Independence
Patients Who Have Irregular Astigmatism
Patients Who Have Poor Visual Potential
Patients Who Have Weak Zonules
Preoperative Measurement and Calculation Pearls
Box 51.1 Important Measurements and Devices to Use
Determining Axis Alignment
Determination of Anticipated Surgically Induced Astigmatism
Posterior Corneal Astigmatism
Treatment of Residual Astigmatism After Toric IOL Placement
Toric IOL Rotation
What Is the Current Location of the Toric IOL?
Ento Key
Fastest Otolaryngology & Ophthalmology Insight Engine
